Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Most commercial photovoltaic cells (in particular solar cells) today are made from typically 10×10 cm2 up to 21×21 cm2 multicrystalline (mc) silicon wafers which are cut from a cast multicrystalline silicon block. The main processing steps on forming a solar cell after cutting the silicon wafer in the most widely adopted screen printed solar cell process are: 1) surface damage etch, 2) texturing, 3) diffusion, 4) SiN deposition, 5) screen printing of metal contacts, 6) firing, 7) edge isolation, and 8) electrical characterisation and binning. More sophisticated solar cell concepts use so-called selective emitter structures in which highly doped local areas are formed under metal contacts. Other advanced cell concepts use point contacts on the rear to improve the rear surface recombination. Normally, the electrical performance of a cell is measured only towards or at the end of the production process.
The initial wafers are normally produced by sawing a large cast silicon block (also known as an ingot, typically up to 1×1×0.7 m3 in size) into square (10×10 cm2 up to 21×21 cm2) shaped columns (also known as bricks), which are then wire sawn into individual wafers (each typically 150-300 μm thick). Currently, some wafer manufacturers use minority carrier lifetime measurements such as quasi steady state photoconductance or photoconductance decay measurements along the edge of the square block or brick to obtain information about the local material quality. One or several line-scans within the wafer area of individual wafers can also be used to assess the wafer quality. Normally, only limited two-dimensional information about lateral variations of the material quality within each wafer is obtained. This is as a result of a high volume solar cell production line typically handling 1 wafer every one to three seconds, which limits the time available for characterisation.
Some individual solar cell manufacturing processes such as screen printing and firing of electrical contacts can be performed as actual in-line processes, where the partially processed wafers are transported through the process one by one, typically on a belt. Other processes such as diffusion and SiN deposition are often carried out as batch processes with tens or hundreds of wafers being processed simultaneously.
The average throughput of a typical silicon solar cell production line may be one solar cell every one to three seconds, which limits the time available for in-line characterisation of each sample. Existing spatially resolved measurements are generally too slow to yield high resolution two-dimensional information about the electronic wafer quality in such short timeframes. On the other hand it is known that small defects can have a large impact on device performance. High spatial resolution (<1 mm per pixel) is thus required for a reliable characterisation. Manufacturers thus have limited tools that allow them to characterise the electronic properties of every wafer or of even a large fraction of wafers going through a production process with sufficiently high lateral spatial resolution.
Certain materials that emit luminescence have a gap in their electronic density of states, the so-called bandgap. Such materials are referred to as bandgap materials. Direct and indirect bandgap semiconductors, including silicon, are included in this definition. Dislocations are a common type of structural defect in semiconductors such as silicon, and their presence strongly affects the electronic properties of the materials and consequently the performance of devices such as solar cells manufactured from them.